Student project (£100 budget) to build a submersible based around a magnetohydrodynamic drive engine.
The thing about this sort of engine is that it has no moving parts!
It uses the principle that a magnetic field can induce currents in a conductive liquid. Or if you induce a current in a conductive liquid and in the presence of a strong magnetic field, then the liquid can act as the moving part of an electric motor. Moving the liquid in one direction and the vehicle (motor) moves in the other.